Page 326 - Introduction to Paleobiology and The Fossil Record
P. 326
SPIRALIANS 1: LOPHOPHORATES 313
the gape of the two shells. Thus an increased brachiopod provincialism generally decreased
volume of nutrient-laden fl uid may fl ow into during the period. Provinciality was most
the mantle cavity while grains of sediment marked during the Early Ordovician, when a
with diameters exceeding the shell gape will range of platform provinces associated with
still be excluded. So far so good. the continents of Baltica, Gondwana, Lauren-
During the Permian, a group of aberrant tia and Siberia (see Appendix 2) were supple-
productoids, called the richthofeniids, mim- mented by centers of endemism associated
icked corals and built biological frameworks with a range of microcontinents and volcanic
that may be found as fossils in the Salt Ranges arcs and island complexes.
of Pakistan and the Glass Mountains of Texas. Provincialism was reduced during the Silu-
These brachiopods have a cylindrical pedicle rian with the close proximity of many major
valve attached to the substrate and a small, continents. By the Wenlock, however, two
cap-like brachial valve. It is diffi cult to under- broad provinces, the cool-water Clarkeia and
stand how these animals fed. A possible sce- the mid-latitudinal Tuvaella faunas, empha-
nario involves the flapping of the upper, sized an increasing endemism, climaxing
brachial valve to generate currents through the during the Ludlow and Prídolí epochs. Pro-
brachiopod’s mantle cavity. Rudwick fi lmed vinciality was particularly marked during the
the flow of water through the cylindrical, Mid Devonian coincident with peak diversi-
lower, pedicle valve as the upper valve was ties in the phylum. Clear biogeographic pat-
moved up and down. Fluid did in fact move terns continued into the Carboniferous, but
efficiently through the animal, bringing in the Permian was characterized by higher
nutrients and flushing out waste. The para- degrees of provinciality probably associated
digm, however, failed the test of fi eld-based with steep climatic gradients.
evidence. Specimens of the athyride Compos- During the Triassic, brachiopod faunas, fol-
ita apparently in life position occur attached to lowing an interval of cosmopolitan disaster
the upper valve of the richthofeniid. Vigorous taxa, became organized into Boreal (high-lati-
flapping of the valve was thus unlikely and it tude) and Tethyan (low-latitude) realms (Box
would not have been an ideal attachment site 12.5). This pattern continued throughout the
for an epifauna. Rather, these aberrant animals Mesozoic, but with centers of endemism and
may have developed lophophores with a ciliary occasional modifications due to ecological
pump action to move currents through the factors such as the circulation of ocean currents
valves. One hypothesis has been rejected, and and the local development of chemosynthetic
another stands as a possibility – we cannot environments. Biogeographic patterns among
prove how the richthofeniid brachiopods func- living forms reflect their Cenozoic roots: a
tioned, but the paradigm approach offers a southern area, the northern Pacifi c, and a
reasonably objective way for paleontologists northern area (Atlantic, Mediterranean, North
to approach these problems. Sea and the circumpolar northern oceans) are
based on a variety of articulated brachiopod
associations. The linguliformeans have more
Distribution in space: biogeography
widespread, near-cosmopolitan distributions.
The biogeographic patterns of the linguli-
formean brachiopods were quite different
from those of the craniiformeans and rhyn- BRYOZOA
chonelliformeans. The former had planktotro- Besides these, there were the Bryozoa, a
phic larval phases (see p. 241) with a facility small kind of Mollusk allied to the Clams,
for wide dispersal; in contrast the lecithotro- and very busy then in the ancient Coral
phic larvae of the latter were short-lived and work. They grew in communities, and
thus individual species were less widely distrib- the separate individuals are so minute
uted. Cambrian brachiopods were organized that a Bryozoan stock looks like some
into tropical and polar realms. Linguliformeans delicate moss. They still have their place
developed widespread distributions in shelf among the Reef-Building Corals, but
and slope settings; rhynchonelliformeans were play an insignificant part in comparison
more diverse in the tropics, preferring shallow- with that of their predecessors.
water carbonate and mixed carbonate-
siliciclastic environments. In the Ordovician, Atlantic Monthly (April, 1863)
